The invention relates to a color display tube comprising a display window with a circumferential upright edge and corner areas, a color selection electrode comprising corner sections to which suspension means are coupled, which color selection electrode is suspended from supporting elements having free end portions, secured to the corner areas.
The invention also relates to a supporting element for use in a color display tube, a corner section for use in a color selection electrode of a color display tube, and a color selection electrode provided with such a corner section.
A color display tube as described in the opening paragraph is disclosed in U.S. Pat. No. 5,003,218. The color display tube according to this specification is provided with a color selection electrode having a frame consisting of four diaphragm parts and four corner sections, suspended in the corners of the display window.
The color display tube described in U.S. Pat. No. 5,003,218 is provided with a color selection electrode to make sure that electron beams coming from three electron guns, mounted in a neck portion of the tube, only excite one color of electroluminescent material on the inner side of the display window. This color selection is achieved by applying, for instance, a shadow mask in the tube. This mask has a large number of apertures, in most cases arranged in either a slotted or a dotted pattern. If the color selection electrode is not positioned in the color display tube in a stable manner, small deviations of its position will lead to a deterioration of the picture quality. When the color selection electrode is shifted slightly, the shadowing effect of the color selection electrode changes and, consequently, the electron beams do not hit the appropriate electroluminescent material on the display window. This misregistration causes a lack of the corresponding color, or even worse, the wrong color of electroluminescent material is excited. These misregistrations cause discoloration of the display tube that lead to a deterioration of the quality of the picture on the color display tube. In practice, color display tubes provided with the suspension system as described in U.S. Pat. No. 5,003,218 show discolorations that are too large to fulfil the ever-increasing demand for picture quality. Especially wide-screen tubes and tubes with a real flat or almost flat outer surface of the display window suffer from these problems. It is a disadvantage of the known color display tube that it shows too large misregistrations.
It is an object of the invention to provide a color display tube having a color selection electrode with an improved suspension system as compared with the type described in the opening paragraph, which strongly diminishes the registration errors on the display window.
According to the present invention, this object is realized with a color display tube which is characterized in that the free end portions have a diameter which is smaller than 5 mm for display windows having a width of more than 600 mm and smaller than 4 mm for display windows having a width of less than 600 mm.
The invention is based on the recognition that the registration errors are diminished when the friction between the suspension means and the supporting elements is decreased. This is realized when the diameter of the free end portions is reduced. The lower friction enhances the positional accuracy of the color selection electrode with respect to the display window.
During the manufacturing process, the color selection electrode has to be inserted into and extracted from the display window several times. First, the black matrix layer and then the three phosphors are applied by using a photosensitive process. For these four process steps, the color selection electrode has to be inserted and extracted. The insertion reproducibility is a major factor in the positional accuracy of the color selection electrode. The color selection electrode will take up about the same position when it is inserted again if the friction is low, thereby improving the insertion reproducibility.
A second source of misregistration is shifts of the color selection electrode that may occur in the color display tube after it has been manufactured. These shifts are caused by heavy shocks to which the color display tube is exposed, for instance during transportation. For this situation as well, a lower friction between the suspension means and the supporting elements leads to a larger positional stability of the color selection electrode. Because of the lower friction, the color selection electrode returns more easily to the original position after a shock has occurred.
In larger tubes, especially wide-screen tubes with a slotted mask, the misregistrations are mainly caused by shifts of the color selection electrode in the east/westxe2x80x94normally the horizontalxe2x80x94direction. This causes the width of the tube to be the dominant factor for determining the diameter of the free end portion of the supporting means. Due to the weight of the tube and the associated color selection electrode for larger tubesxe2x80x94i.e. display windows width of more than 600 mmxe2x80x94a larger diameter of the free end portion is preferred.
The free end portions normally used in color display tubes are in general rotationally symmetric around some given axis. This makes it possible to define a diameter for free end portions, namely, the diameter of the smallest cylinder which contains the free end portion and whose axis coincides with the axis of the supporting element. This definition is not limited to a free end portion which is spherically curved; amongst others, also cylindrical, conical and other rotationally-symmetric shapes are included.
A preferred embodiment of the color display tube according to the present invention is characterized in that the diameter of the free end portion is smaller than 4 mm for display windows having a width of more than 600 mm and smaller than 3 mm for display windows having a width of less than 600 mm. A further decrease of the diameter of the free end portions further lowers the friction between the supporting elements and the suspension means. This preferred embodiment yields an excellent compromise between the friction and the mechanical strength of such a free end portion.
A further embodiment is characterized in that the free end portions are substantially spherically curved. From a manufacturing point of view, a supporting element with a free end portion which is spherically curved is relatively easy to produce. For instance, if an assembledxe2x80x94two-partxe2x80x94supporting element is used, the free end portions may be obtained by making use of a highly accurate standard ball.
A still further embodiment is characterized in that the suspension means comprise a resilient element which exerts a force of less than 6 newtons on the free end portions. Forces smaller than 4 newtons are preferred.
This measure also decreases the friction between the suspension means and the free end portions of the supporting elements. A color display tube with a color selection electrode comprising a frame with corner sections and diaphragm parts allows the construction of a lightweight frame. This makes it possible to use low forces between the resilient elements of the suspension means and the free end portions of the supporting elements, thereby decreasing the friction. In the situation where the forces between the suspension means and the supporting elements are less than 6 newtons, the misregistrations become so small that a picture with a good color purity can be reproduced on the color display tube. Misregistrations caused by friction between the suspension means and the supporting elements can even be ignored if the forces are less than 4 newtons.
The invention also relates to the supporting elements for use in such a color display tube, the corner section of a color selection electrode, and a color selection electrode provided with such a corner section.
These and other aspects of the invention are apparent from and will be elucidated by way of non-limitative examples with reference to the drawings and the embodiments described hereinafter.